Tunable Fabry-Perot filters operating in the 3 to 5 μm range for infrared micro-spectrometer applications

In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures which distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical tratismission peak shifted from 4.5 mu m to 3.6 mu m. Without antireflection coating at the back of the silicon substrate -50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is similar to 20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.